Accumulation of serum lipids by vascular smooth muscle cells involves a macropinocytosis-like uptake pathway and is associated with the downregulation of the ATP-binding cassette transporter A1
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Vascular smooth muscle cells (VSMC) are present in arterial intima before atherosclerotic plaques develop and are likely to be exposed to unmodified serum lipids as they enter the vessel wall. We examined the effects of sera from mice on the morphology and function of mouse VSMC. Incubation of a mouse VSMC line (MOVAS) with sera from normocholesterolemic (C57BL/6J) or hypercholesterolemic (APOE−/−) mice caused concentration-dependent increases in lipid accumulation as measured by AdipoRed, with the extent of lipid uptake significantly greater with the latter sera type. Inhibition of c-Jun N-terminal kinases (SP600125), Src kinases (AG1879), and clathrin-dependent endocytosis (monodansylcadaverine) to disrupt scavenger receptor-mediated uptake of lipids had no effect on serum-induced lipid accumulation by VSMC. By contrast, inhibition of macropinocytosis with antagonists of PI-3 kinase (LY294002) and actin (cytochalasin D) markedly reduced lipid accumulation. Serum exposure reduced the expression of the ATP-binding cassette transporter A1, consistent with impaired cholesterol efflux, but had no effect on the expression of markers of VSMC differentiation. Moreover, the expression of several inflammation and foam cell markers was unchanged (CCL2, CCL5, and CD68) by mouse sera. The accumulation of unmodified serum lipids by VSMC involves a macropinocytosis-like uptake pathway and is associated with the downregulation of the ATP-binding cassette transporter. We speculate that VSMC may play an atheroprotective role in arterial intima by acting as a “sink” for unmodified lipids.
KeywordsSerum lipids Vascular smooth muscle cells Foam cells Macropinocytosis ATP-binding cassette transporter A1
The authors would like to thank Mr. Stephen Firth for his expertise and assistance in acquiring the time-lapse videos of the VSMC. This work was supported by research grants from the National Health and Medical Research Council of Australia (NHMRC; ID nos. 545942 and 436825). JR was supported by a scholarship from the Monash University Faculty of Medicine, Nursing and Health Sciences, while GRD and CGS were supported by fellowships from the NHMRC (ID nos. APP1006017 and 350327, respectively).
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